Fitted parameter values with single-substrate experiments
Substrate | P-gp Efflux to Apical Chamber k2a | P-gp Dissociation to Apical Membrane krb | P-gp Binding Constant KC (Cytosolic Dissociation Constant KD, Aq)c | Partition Coefficient KPCd | Steady-State Passive Permeability Coefficientse | Vmax/Km | |||
|---|---|---|---|---|---|---|---|---|---|
| Basolateral Transporter (Passive Transport into Cytosol)f | Apical Transporter (Passive Transport into Cytosol)f | ||||||||
| s -1 | M -1 | nm/s | s-1 (%) | ||||||
| AMP | 150 | 2 × 106 | 1000 (5 μM) | 200 | PBA = 420 ± 50 | 0 (0) | 0 (0) | ||
| PAB = 400 ± 50 | |||||||||
| QND | 10 | 1 × 105 | 10,000 (0.1 μM) | 700 | PBA = 500 ± 100 | 0 (0) | 0 (0) | ||
| PAB = 500 ± 100 | |||||||||
| LPM | 1 | 1 × 105 | 20,000 (0.01 μM) | 3000 | PBA = 350 ± 80 | 100 ± 20 (40) | 0 (0) | ||
| PAB = 400 ± 100 | |||||||||
| DGX | 10 | NDg | NDg | NDg | PBA = 30 ± 8 | 30 ± 10 (70) | 2 ± 1 (5) | ||
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| PAB = 25 ± 5 |
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AMP, amprenavir; QND, quinidine; LPM, loperamide; DGX, digoxin; ND, not determined.
↵ a Estimate for the efflux rate constant k2, from P-gp into the apical chamber, given by the ratio of the fitted k2T0 for each drug and the center of the box value of T0 = 2 × 10-4 M.
↵ b Estimate for the dissociation rate constant kr, from P-gp back into the inner apical monolayer.
↵ c The binding constant between P-gp and the inner apical monolayer shown is the average of the best fits (> 160 for each drug concentration). Below each binding constant, we show in parentheses the appropriate dissociation constant for each drug relative to the aqueous phase, calculated as KD, Aq = 1/(KC * drug partition coefficient [phosphatidylserine (PS)/phosphatidylethanolamine (PE)/cholesterol (chol)]), the liposome mimic for the inner apical monolayer.
↵ d Equilibrium drug partition coefficient to 0.1 μ m PS/PE/chol (1:1:1) liposomes. This lipid composition is a rough mimic for the inner apical monolayer, as described in Tran et al. (2005).
↵ e Steady-state passive permeability coefficients measured across the confluent cell monolayer in the presence of the P-gp inhibitor GF120918. PBA and PAB are shown. The values are not always symmetric.
↵ f The fits for the other transporters. Both Vmax and Km were independently fitted, but only their ratio was constant. Transport is the fraction of transport into the cytosol, B>C or A>C, due to the transporter, relative to all +GF120918 passive transport.
↵ g The mass action equations only fit the product KC * KPC, which for digoxin is ∼5 × 104 M-1. This value is similar to that for amprenavir. The partition coefficient for digoxin has not been measured, so we cannot separately estimate the binding constant, KC, to P-gp or the dissociation rate constant kr = k1/KC.